The present invention relates to a method for stabilizing the alpha acid or humulone content in hops prior to, during, and subsequent to the processing of hops for use in brewing. This invention also pertains to the isomerization of hops or hop extracts with good efficiency to obtain high yields of isomerized alpha acids which are useful as constituents in brewing.
The lupulin or humulin is the glandular trichomes of the strobiles of hops which are used for making various beverages. The lupulin contains two important constituents which are commonly used in brewing beer. These constituents are humulones, also referred to as alpha acids, and lupulones, sometimes referred to as beta acids. The humulones are the primary bitter constituent of the hops which are utilized in brewing beer.
Humulones or alpha acids tend to undergo isomerization forming isohumulones or iso-alpha acids. These isomerization products are desirable constituents in brewing and, thus, isomerization is a preferred process reaction. Isomerization typically is carried out intentionally in the brewing kettle.
Humulones are also subject to certain undesirable phenomena, however. Humulones tend to undergo deterioration and break down with undesirable by-products. For instance, under ambient conditions humulones or alpha acids have a tendency to oxidize and polymerize into hard resin. Moreover, during isomerization by-products may form, e.g., humulinie acids, which waste the humulone or alpha acid content of the hops. Another problem is the interreaction of humulones with other constituents in hops or hop extracts, such as lupulones or beta acids, under the stringent operating conditions of prior art processing methods.
The most simple method for preparing hops for use in brewing consists of introducing baled hops directly into the brewing kettle and brewing the mixture of malt, hops and water at the boiling temperature and at acidic, or lower, values of pH. During this procedure, the alpha acids present will isomerize. As a result of the aformentioned undesirable phenomena, however, this technique which is one of the oldest in the brewing business results in only a poor conversion of alpha acids to iso-alpha acids, i.e., in the order of only 25-35% isomerization of the alpha acids.
To prevent or at least ameliorate the deterioration of humulones or alpha acids and undesirable side reactions, most hops processing methods include steps which relate to (1) stabilizing the hops or hop extracts against the deterioration of alpha acids and (2) converting the alpha acids to isomerized alpha acids while minimizing the occurrence of side reactions and by-product formation. These methods have as objects increasing the amount or concentration of alpha acids available for isomerization and increasing the yield of isomerized alpha acids.
For instance, to protect the hops from deterioration, they are sometimes stored at reduced temperatures and/or under vacuum or in an inert atmosphere, e.g., nitrogen. This is an expensive procedure, however.
It has also been proposed to concentrate the alpha acids by separating them from the rest of the hops constituents and charging the concentrated alpha acids to the brewing kettle, e.g., Naatz, U.S. Pat. No. 2,833,652, Muller, U.S. Pat. No. 3,275,447, and Hokanson et al., U.S. Pat. No. 3,787,586, incorporated herein by reference. As a result of using a concentrated feed, the yield of isomerized alpha acids is higher. The process remains inefficient, however.
Still other techniques involve chemically treating separated alpha acids to convert them to iso-alpha acids prior to their addition to the brewing kettle. Such techniques are described in Clarke et al., U.S. Pat. No. 3,765,903 and Koller et al., U.S. Pat. No. 3,952,061. In general, these processes relate to the use of complicated procedures wherein the hops are first extracted with organic solvents and the solubilized hop extract containing alpha acids is then treated with alkali to convert the alpha acids present to their isomerized form. Using this technique, conversions of 80-85% of the recovered alpha acids to iso-alpha acids have been reported.
A shortcoming of these procedures, however, is the difficulty encountered with recovering both the non-isomerized and isomerized alpha acids from the organic solvent. Thus, while the alpha acids may undergo an 80-85% conversion to the isomerized products, as much as 30% or more of the alpha acids, based on the amounts originally present, can remain unrecovered and lost. Moreover, the use of solvent extraction is an expensive procedure.
Other methods have also been proposed, such as the well-known technique of treating the lupulin glands with methylene chloride, after grinding, which tends to separate and concentrate the alpha acids from the rest of the hops so as to yield more of the desired starting material for subsequent use in brewing.
The present invention overcomes many of the shortcomings of the previously known methods for stabilizing hops and obtaining high yields of isomerized product. Some of the advantages of this invention include: compatibility with previously known procedures for processing hops in brewing; stabilization of alpha acids in hops to prevent or sharply reduce deterioration thus resulting in greater economy in storage and packaging, as well as longer shelf life; and the avoidance of large amounts of organic solvents, which are expensive and toxic and result in a loss of alpha acids in recovery from the solvent.
In addition to stabilizing the alpha acid content of hops against deterioration, the method of this invention enables hops to be isomerized at faster reaction rates, under mild reaction conditions, with fewer losses of alpha acids to side reactions, thus yielding higher conversions of the alpha acids to the desired isomerization products.